A 'terror of tyrannosaurs': the first trackways of tyrannosaurids and evidence of gregariousness and pathology in Tyrannosauridae

Abstract

The skeletal record of tyrannosaurids is well-documented, whereas their footprint record is surprisingly sparse. There are only a few isolated footprints attributed to tyrannosaurids and, hitherto, no reported trackways. We report the world's first trackways attributable to tyrannosaurids, and describe a new ichnotaxon attributable to tyrannosaurids. These trackways are from the Upper Cretaceous (Campanian - Maastrichtian) of northeastern British Columbia, Canada. One trackway consists of three tridactyl footprints, and two adjacent trackways consist of two footprints each. All three trackways show animals bearing southeast within an 8.5 meter-wide corridor. Similarities in depth and preservation of the tyrannosaurid tracks indicate that these three trackways were made by track-makers walking concurrently in the same direction. These trackways add significantly to previous osteology-based hypotheses of locomotion and behavior in Tyrannosauridae by providing ichnologic support for gregariousness in tyrannosaurids, and the first record of the walking gait of tyrannosaurids.

Figure 1. Trackway map.

A portion of the tracksite showing three parallel trackways of Bellatoripes fredlundi and a partial trackway of a smaller theropod Saurexallopus cordata. Trackway A: PRPRC 2011.11.001; Trackway B: PRPRC 2012.04.002; Trackway C: PRPRC 2012.04.003. This trackway map was produced by tracings taken from the site, but outlines of the prints were produced by studying the silicone moulds of all the trackways at the Peace Region Palaeontology Research Centre (PRPRC). Trackway B has two tracks (only the first one is figured, the second is a pace length in front), but a debris slide covered the second print shortly after discovery at the end of the field season.

Figure 2. In situ Trackway A images.

a) Print #2 of Trackway A (in situ) - PRPRC 2011.11.001 (right); b) Trackway A (in situ) view to the east of prints #1–3. Note the thick layer of kaolinite in the freshly excavated area in front of print #3.

Figure 3. Excavation of Tracksites (A: Trackway A; B: Trackway B; C: Trackway C).

View toward the north from near Trackway C (foreground with LGB excavating and silicone mould of print #1 at the bottom of the figure), Trackway B is located in the center of the image and the silicone mould of Trackway B, print #1 is visible. Trackway A is covered in a green tarp near the top left of the image. The headings of the trackways are from left to right with other tracks likely buried by sediments forming a steep cliff.

Figure 4. Methods of trackway and footprint measurements used in this study. DIV

, digit divarication; DL, digit length; FDL, free digit length; FL, footprint length; FW, footprint width; PL, pace length; SL, stride length.

Figure 5. Footprint length (digit III) versus hip height (h, leg length) of Albertosaurus, Gorgosaurus, and Daspletosaurus.

Graphical results and best-fit regression line for footprint length (FL, calculated at the proximodistal lengths of digit III phalanges) compared to leg length as calculated from the sum of the proximodistal femur, tibia and astragalus, and metatarsal III lengths of late Campanian - early Maastrichtian tyrannosaurids Albertosaurus, Gorgosaurus, Daspletosaurus , . All data are in millimeters (mm), and are unadjusted . Five percent was added to all totaled lengths to account for anatomical unknowns. Standard error for footprint length +/–33.3 mm; hip height +/–129 mm.

Figure 6. Digit III length versus femur length for Albertosaurus, Gorgosaurus, and Daspletosaurus.

Graphical results and best-fit line for comparing footprint length FL (digit III length, as calculated by totaling the proximodistal lengths of digit III phalanges and adding 5% to the total length to account for anatomical unknowns) to osteologic femur length (y) for late Campanian - early Maastrichtian tyrannosaurids Albertosaurus, Gorgosaurus, Daspletosaurus . The calculated y can then be used to estimate age of the track-maker using the methods of Erickson et al. , . All data are in millimeters (mm), and are unadjusted . Standard error for footprint length +/–32.3 mm; femur length +/–55.7 mm.

Figure 7. Photogrammetic image of Bellatoripes fredlundi Holotype (Trackway A).

Figure rendered from images of the silicone mould (PRPRC 2011.01.001M). Lateral view (top) and plan view (bottom). Note that the topographic profile for the lateral view is reversed in this orientation. Topographic profile scale and linear scale are in meters.

Figure 8. Comparison of (Footprint Area to Length Ratio).

Bellatoripes fredlundi holotype, Trackway A, Print #2 PRPRC 2011.11.001 (left) Footprint Area to Length Ratio = 24.3 cm²∶1 cm. Irenesauripus mclearni track from the Gates Formation (right) Footprint Area to Length Ratio = 8.4 cm²∶1 cm.

Figure 9. Trackway B of Bellatoripes fredlundi.

a) Partially excavated print #2 of Trackway B (not mapped or moulded); b) Photogrammetric image of Trackway B, print #1 from its silicone mould (PRPRC 2012.04.002); c) Photogrammetric image of Trackway C, prints #1 and 2 from its silicone mould (PRPRC 2012.04.003). Topographic profile scales and linear scales are in meters.

Figure 10. Striations on Bellatoripes fredlundi paratype track.

a) Photograph of silicone mold of print #2 Trackway C (PRPRC 2012.04.003), arrows pointing to areas with striations; b) photograph of striations on caudal drag marks leading up to print #2; c) photograph of striations on the outer margin of digit II print #2 Trackway C.

Figure 11. Hypothesized pes movement of the track-maker for Bellatoripes fredlundi.

The digits were not dragged cranially through the substrate as previously described in theropods footprints , , . The pes and digits were retracted from the substrate along an opposite trajectory of their entry prior to the pes moving forward in the next step cycle. Arrows indicate trajectory of the foot of Trackway C, print #2 as deduced from entry striations at the point of pes entry (left frame) and exit (center frame) into the substrate.